1. Field of the Invention
The present invention relates to a Bi-CMOS integrated circuit including a vertical type PNP bipolar transistor capable of operating at high speed.
2. Description of the Related Art
Conventionally, in Bi-CMOS integrated circuits, bipolar devices and CMOS devices have been formed on the same chip so that the driving capability of the CMOS devices are enhanced by the bipolar devices in order to obtain a high responsive CMOS. Although the Bi-CMOS integrated circuits have exhibited both bipolar and CMOS characteristics, they have not been able to give excellent functions such as high speed operation, high integration, high driving capability, low power consumption and the like.
A process of fabricating Bi-CMOS integrated circuits will be described hereinafter with reference to FIGS. 4(a) to (e).
As shown in FIG. 4(a), a first N-type buried layer 105 having a sheet resistance of 50 .OMEGA./.quadrature. is formed on an NPN bipolar transistor forming region 101 of a P-type silicon substrate (hereinafter referred to as substrate) 100.
Also formed on a vertical type PNP (hereinafter referred to as V-PNP) bipolar transistor forming region 102 are the N-type first buried layer 105 and a P-type second buried layer 106 having a sheet resistance of 350 .OMEGA./.quadrature. which is isolated from the substrate 100 by the N-type first buried layer 105.
The N-type first buried layer 105 is also formed on a P-channel MOS transistor forming region 103 and the P-type second buried layer 106 is also formed on an N-channel MOS transistor forming region 104. The first isolation layers 107 are formed on the substrate 100 for isolating an element in the NPN bipolar transistor forming region 101 and an element in the V-PNP bipolar transistor forming region 102 respectively at the same time that the P-type second buried layer 106 is formed using a known photolithography technique and an ion implantation and an annealing technique.
Subsequently, as shown in FIG. 4(b), an N-type epitaxial film 108 having the N-type impurity concentration of 1.5.times.10.sup.16 atoms/cm.sup.3 is deposited and grown in the thickness of 2 to 4 .mu.m on the surface of the substrate 100 using an epitaxial technique. Then, as shown in FIG. 4(c), there are formed on the N-type epitaxial film 108 a second isolation layer 109 for isolating elements and a P-type region (hereinafter referred to as P-region) 110 for forming an N-channel MOS transistor thereon respectively having surface concentration of 2E16 atoms/cm.sup.3 and diffusion depth of 1-2 .mu.m so that the previously buried first isolation layers 107 and second isolation layers 109 are connected with each other, and the P-region 110 and the P-type region 111, formed by the out diffusion of second buried layer 106 in the device forming region 104 are connected with each other.
Whereupon, inasmuch as the first isolation layers 107 are formed of antimony and the second isolation layers 109 are formed of boron, the second P-type buried layer 106 is diffused more deeply upward in the N-type epitaxial film 108 than the N-type first buried layer 105 at the V-PNP bipolar transistor forming region 102. As a result, over the first N-type buried layer 105 there is formed a P-type buried layer 112 serving as a collector of the V-PNP bipolar transistor 102 isolated from the substrate 100.
Thereafter, LOCOS oxide films 113 are formed due to isolation of MOS device in the thickness of about 7000 angstroms by the LOCOS (localized oxidation of silicon) method on the substrate as shown in FIG. 4(d) which has therein the second isolation layers 109, the P-region 110 and the second buried layer 111.
Then, a layer 114 forming a collector contact of the NPN bipolar transistor is formed by diffusion of the N-type impurity. Successibly, a layer 115 for forming a collector contact of the V-PNP transistor is formed by diffusion of the P-type impurity to reach the P-type buried layer 112.
Then, a P-type layer 116 for forming a base of the NPN transmitter is formed with a surface concentration of 5E17 atoms/cm.sup.3, and in a diffusion depth of 0.6 .mu.m.
As shown in FIG. 4(e), on the substrate set forth above a gate oxide film 117 and a polysilicon gate 118, using a known MOS gate forming technique, are formed for the MOS transistors.
Then, there are formed a layer 119 for forming an emitter of the NPN bipolar transistor and a layer 120 for forming a base contact of the V-PNP transistor and a layer 121 for forming a source and a drain (hereinafter referred to as source/drain) of the N-channel MOS transistor using an arsenic implantation technique.
There are formed a layer 122 for forming a base contact of the NPN bipolar transistor, a layer 123 for forming an emitter of the V-PNP bipolar transistor and a layer 124 for forming a source/drain of the P-channel MOS transistor using a BF.sub.2 ion implantation technique.
Thereafter, the Bi-CMOS structure is completed by opening contact holes for wiring each of the elements and applying aluminum electrodes using a known technique whose detailed description is omitted.
The Bi-CMOS integrated circuit fabricated in the process set forth above has a concentration characteristics as illustrated in FIG. 5 and a high-frequency characteristics as illustrated in FIG. 6. That is, inasmuch as the V-PNP bipolar transistor has a base layer formed by the N-type epitaxial film having a uniform concentration as shown in FIG. 5, the high-frequency characteristics thereof, i.e., a cut-off frequency f.sub.T becomes about 200 MHz, which results in reducing the operation speed at one tenth or less of the other Bi-CMOSs.
Since the base region in the PNP bipolar transistor is formed of the epitaxial layer having a uniform concentration, the minority carriers injected from the emitter region to the base region are not subjected to an electric field aiding effect.
Accordingly, there was a problem that the Bi-CMOS integrated circuit is deteriorated in its f.sub.T -IC characteristic compared with the PNP bipolar transistor having a diffused base which is known by the following equation. EQU 1/2.pi.f.sub.T =.tau.e+.tau.b+.tau.x+.tau.c (1)
where a second term .tau.b (base time constant) is largest and is expressed as: EQU .tau.b=W.sub.B.sup.2 /nD.sub.B ( 2)
where W.sub.B is a base width, n is a constant dependent on minority carrier distribution in the base and D.sub.B is a diffusion constant of the minority carrier concentration in the base.
The epitaxial PNP bipolar transistor is a transistor having normally a uniform base so that it is not influenced by the electric field aiding effect caused by a diffusion gradient. As a result, the base time constant of the epitaxial PNP bipolar transistor .tau.b is greater than but the cut-off frequency f.sub.T thereof is less than those of the transistor having the diffused base.
Another fabrication process has been proposed to solve the problem set forth above.
Firstly, in FIG. 4(f) over the surface of the substrate 100 as illustrated in FIG. 4(a), the N-type epitaxial film 108 having an N-type impurity concentration of 1.5E 15 atoms/cm.sup.3 is formed in the depth of 2 to 4 .mu.m. Secondly, second isolation layers 109 for isolating each element and a P-region 110 for forming the N-channel MOS transistor are formed at the same time with the surface concentration of 2.times.10.sup.16 atoms/cm.sup.3 in the depth of 1 to 2 .mu.m, then the first isolation layers 107 are connected to the second isolation layers 109 and the second buried layer 111 is connected to the P-region 110. By such an arrangement, over the first buried layer 105 at the V-PNP bipolar transistor forming region 102 there is formed the P-type buried layer 112 serving as the collector of the V-PNP bipolar transistor.
Thereafter, an N-type diffusion region (hereinafter referred to as N-region) 125 for forming the P-channel MOS transistor is formed and the same N-region 125 having the surface concentration 5.times.10.sup.16 atoms/cm.sup.3 with the diffusion depth of 1 to 2 .mu.m is formed at the V-PNP transistor bipolar transistor forming region 102 to reach the P-type buried layer 112.
Then, there are formed, as shown in FIG. 4(g), the LOCOS oxide film 112, the N-type layer 114 for forming the collector contact of the NPN bipolar transistor and the base layer 116 of the NPN bipolar transistor in the same manner as in FIG. 4(d).
Thereafter, in the same way as in FIG. 4(e), there are formed respectively the gate oxide film 117, the polysilicon gate 118, the emitter layer 119 of the NPN bipolar transistor, the layer 120 for forming the base contact of the V-PNP bipolar transistor, the source/drain 121 of the N-channel MOS transistor, the layer 122 for forming the base contact of the NPN bipolar transistor, emitter layer 123 of the V-PNP bipolar transistor and the source/drain 124 of the P-channel MOS transistor.
According to the process set forth above, the V-PNP bipolar transistor can be formed in the N-region which is formed by the diffusion technique so that the cut-off frequency f.sub.T is improved. However, the N-type epitaxial film should have the low concentration, i.e. the N-type impurity concentration of 1.5.times.10.sup.15 atoms/cm.sup.3 to form the due to the MOS characteristics and the concentration of the N-region should be sufficiently lower than the same value in view of the process margin.
However, in the case of the Bi-CMOS integrated circuit including the V-PNP bipolar transistor, the surface concentration of the P-type second buried layer 111 formed of boron should be high and the area thereof should be large in view of the wafer so that an auto-doping phenomenon occurs in the wafer per se when the wafer epitaxial film is formed. Therefore, it was difficult to control the N-type specific resistivity of the epitaxial film to the low concentration.
Accordingly, when the control characteristics of the concentration of the N-type epitaxial film is improved in the conventional fabrication process, the high frequency characteristics are determined because the concentration of the base of the bipolar transistor becomes uniform, while when the high frequency characteristics are improved, the high frequency characteristics is deteriorated so that the control of the N-type epitaxial film becomes difficult. The relationship between the control of the concentration and the high frequency characteristics is a so-called tradeoff, and hence, quick improvement thereof has been demanded.